Several experiments suggest that energy is stored in the primary photonic event in visual transduction. A crucial test of this hypothesis is that bathorhodopsin can thermally revert in certain cases to rhodopsin. For example, bathoiodopsin and bathobacteriorhodopsin have thermal pathways back to iodopsin and bacteriorhodopsin. A cis-trans isomerization is not an energy storing process and, furthermore, experiments indicate that even though bacteriorhodopsin has spectral transitions which are similar to all rhodopsins, it does not involve a formal cis-trans isomerization. Resonance Raman spectroscopy is providing the first in situ structural information on the retinylidene chromophore. Our spectra indicate that the primary action of light is to delocalize e- density out of the polyene portion of the retinylidene chromophore, thus inducing charge polarization and delta ion charges on the tertiary centers at carbons 5, 9 and 13. However the Schiff base is still protonated in bathorhodopsin and curiously the e- density in the -C double bond HN ion- bond is identical to rhodopsin even though the C double bond C bonds have significantly lower e- density. A relocalization of e-density occurs and the Schiff base is deprotonated by metarhodopsin II. The initial charge polarization can store part of the photon energy and then the positive centers at carbons 5, 9 and 13 appear to induce in milliseconds e- replacement and subsequent ejection of the Schiff base H ions. Thus the cis-trans isomerization is not thecrucial result of the photonic event. In fact, bacteriorhodopsin, which is biologically an energy converter, has evolved to convert light energy efficiently into an ejected H ion without an endoenergetic isomerization. However photoreceptor rhodopsins are not primarily energy converters but quantum detectors and thus must have irreversibility. This we believe is the biological role of the cis-trans isomerization while the role of the ejected H ions appears to be tied to the energy transduction process. BIBLIOGRAPHIC REFERENCES: "Tunable Laser Resonance Raman Spectroscopy as a Probe of Primary Ion Movements in Photoreceptor Cells", A. Lewis, Federation Proceedings 35, 51 (1976). "Preresonance Raman Spectra of Crystals of Retinal Isomers", R.E. Cookingham, A. Lewis, D.W. Collins and M.A. Marcus, J. Am. Chem. Soc. 98, 2759 (1976).